Ten states from Nevada to Texas have weighed in to support two water agencies in their fight with an Indian tribe over control of groundwater in the California desert.

The states filed a brief Monday before the U.S. Supreme Court, which will soon decide whether to take up an appeal by the Desert Water Agency and the Coachella Valley Water District.

The water agencies are challenging a decision by the Ninth Circuit Court of Appeals, which ruled the Agua Caliente Band of Cahuilla Indians has a federally established right to groundwater dating to the creation of its reservation in the 1870s.

If the Supreme Court agrees to hear the case, it would have a rare opportunity to settle the question of whether tribes hold special federal “reserved rights” to groundwater as well as surface water, and to define more clearly the boundaries between state-administered water rights and federal water rights.

The 10 states joined the case in a “friend-of-the-court” brief, saying every state “has an obvious stake in the preservation, maintenance and allocation of their most precious natural resource.”

The states argued the appeals court’s ruling that the tribe has a priority right to groundwater “is literally a watershed opinion washing away the authority and control that states have traditionally exercised over groundwater resources.”

Nevada Attorney General Adam Laxalt led the coalition of 10 states, saying the case is about defending state governments’ authority over the regulation of groundwater…

Other states that signed on in support of the water agencies included Arizona, Arkansas, Idaho, Nebraska, North Dakota, South Dakota, Texas, Wisconsin and Wyoming. California was not among them and has not formally taken a position on the case.

The Agua Caliente tribe sued the water agencies in 2013, seeking to assert rights to groundwater beneath its reservation in Palm Springs and surrounding areas.

The tribe accuses the agencies of imperiling the aquifer by allowing its levels to decline over the years and by using saltier, less pure Colorado River water to replenish the aquifer. The agencies defend their efforts to combat groundwater overdraft and insist that Colorado River water meets all drinking water standards.

Managers of the water agencies argue groundwater is a public resource and the tribe has the same rights under California law as all other landowners to use water pumped from the aquifer.

The Supreme Court hears a small percentage of the cases that are petitioned for review, and the court is expected to announce in the fall whether it will take up the case.

The states’ involvement in the case, and their stance that it presents an important unresolved legal issue, could increase the odds of the court hearing the case.

If the Agua Caliente tribe prevails, the lawsuit would set a powerful precedent for other tribes across the country, strengthening their claims to groundwater.

The U.S. Department of Justice joined the suit in support of the tribe in 2014, saying the federal government has an interest in ensuring water rights for the tribe.

The Supreme Court has never ruled on the question of whether tribes have a federally established right to groundwater.

Groundwater and surface water have long fallen under separate, different water-rights systems.

The case is pushing the courts to sort out how groundwater fits into laws drawn up more than a century ago, before the widespread use of mechanical pumps that enabled people to easily tap underground water supplies.

One of the central legal questions in the case centers on state and federal courts’ varying interpretations of a 1908 Supreme Court decision, Winters v. United States, which affirmed that Indian tribes are entitled to sufficient water supplies for their reservations.

The Ninth Circuit Court of Appeals said the so-called Winters doctrine applies to both surface water and groundwater on federally reserved land – including Indian reservations as well as other lands set aside by the government, such as national forests, national parks and military bases.

The states’ attorneys general pointed out in their legal brief that “as a general matter, water rights must be acquired under state law, even for federal lands.”

They argued that if a reservation created by the federal government “can assert absolute preemption over state groundwater allocation laws and regulations, a state’s effort to effectively manage those limited water resources will be thrown out of balance.”

In a state like Nevada – which has more of its land under federal ownership or control than any other state – the states’ attorneys said the appeals court’s ruling would have “potentially devastating consequences” if it stands. They said claims of priority federal rights would lead to over-allocation of the limited water supplies in western states.

The attorneys said the Ninth Circuit’s decision leaves states “facing a possible tide of federal reserved water right claims,” which they argued creates great uncertainty.

In explaining Nevada’s stance, Laxalt said he has shown throughout his tenure as attorney general that “my office stands ready to defend our state from unlawful federal overreach regardless of the source.”

That position resonates with conservatives in other states. All of the 10 states that sided with the water agencies are led by Republican governors.

Four organizations also submitted briefs supporting the water districts. One of them, the Pacific Legal Foundation, advocates “limited government and the strong protection of private property rights.” It said the Ninth Circuit’s decision could have harmful effects for landowners’ water rights.

The other organizations include the National Water Resources Association, which represents water agencies; the Western Coalition of Arid States, which is comprised of municipal entities; and the Irrigation and Electrical Districts’ Association of Arizona, whose members supply water and power to much of Arizona, including cities as well as most of the state’s farmland.

Those organizations echoed the California water agencies’ concerns, saying if it the appeals court’s decision isn’t overturned, it will threaten established water rights throughout the West, create widespread uncertainty in a region where water is scarce and “force water users to engage in unnecessary litigation.”

The Agua Caliente tribe has until Sept. 6 to submit its argument to the court.

If the tribe wins, the case would continue with other phases to determine whether the Agua Caliente own storage space in the aquifer, whether their rights include a water-quality component and how much groundwater they would be entitled to.

The tribe has about 485 members. Its reservation spreads across more than 31,000 acres in a checkerboard pattern that includes parts of Palm Springs, Cathedral City, Rancho Mirage and surrounding areas.

The tribe owns two golf courses, the Spa Resort Casino in Palm Springs and the Agua Caliente Casino Resort Spa in Rancho Mirage, and has plans to build new subdivisions and another casino. Thousands of homes stand on leased tribal land.

The tribe buys its water from the local water agencies, which operate wells across the Coachella Valley.

Tribal Chairman Jeff Grubbe has said the case is about securing a “seat at the table” for the tribe to have a formal say in decisions about how the aquifer is managed.

“Having a right to govern that water and decide what to do with that water is a right that every tribe should have,” Grubbe told The Desert Sun in an interview last week. He said if the tribe wins, one of the first priorities would be to start treating the Colorado River water that flows to the Coachella Valley and is used to replenish the aquifer.

He said the tribe’s leaders are concerned about the quality of the water and the aquifer’s long-term sustainability, and would be willing to help pay for building treatment facilities to remove dissolved solids and contaminants from the imported water.

The agencies’ managers say the water they get from the Colorado River meets all state and federal drinking water standards. They say treating the water would be expensive and unnecessary. They worry that if the tribe prevails, its privileged rights could drive up water costs for customers and complicate efforts to manage groundwater.

“Our groundwater has been a shared local resource that the public and the tribe can access equally,” said James Cioffi, president of the Desert Water Agency board. He said in a statement that with the tribe now trying to secure special rights, “frankly, we don’t know what exactly they want or how much that could impact our community.”

Leaders of other Native American tribes across the West are closely watching the case.

A list of 35 tribes and five tribal organizations filed a brief in support of the Agua Caliente tribe last year. They included the Spokane Tribe of Indians in Washington and the Pyramid Lake Paiute Tribe in Nevada, as well as other tribes in California.